The Fresh Loaf

News & Information for Amateur Bakers and Artisan Bread Enthusiasts

Question about using refractory cement for the oven floor

mebusybaker's picture

Question about using refractory cement for the oven floor

I am new to this page, however, have cooked in a brick oven many times before.  Now I am building one.  I am attempting to do this as green as possible.  I have had the city dump sidewalk in my yard for the base and am now researching the use of cement vs fire brick for the oven floor.  I have left over red brick from years ago I plan to use as the dome.  The idea is to recycle as much material as possible and keep the build cost to a minimum.  I am wondering whether I could replace fire brick ($$) with cement for the floor.  To eleminate, or rather, decrease the possibility of natural expansion and contraction I have thought of pouring in sawdust or shavings in with the cement as an additional ingredient; technique is very similar to a cob oven.  Any advice out there would be helpful.  Thanks


breadbakin fool's picture
breadbakin fool

It's my understanding that it's not a good idea to use cement, especially portland cement for the hearth baking floor.  It can not handle the intense heat and will break down.  Adding sawdust and shavings to the cement would also be a bad idea, since that would turn it into more of an insulating type of substance and not a heat absorbing substance.  You would have a difficult time getting your hearth up to temperature to insure oven spring, and it would not hold heat for long.  It would be  better to place insulation on top of the slab base, underneath the firebrick hearth. Some designs call for 3 to 4 inches of concrete mass underneath the hearth firebricks to add additional mass and heat storage, but that concrete would not be subjected to the extreme heat of the coals and would probably never get above about 4oo to 5oo degrees, and is just there to provide mass anyway.  You would still need some sort of insulation under that to protect the structural slab from excessive heat, and to enable your hearth to heat up in a reasonable amount of time.  


ClimbHi's picture

I'm no expert on cob ovens, so I'm not sure about the idea to add sawdust to the concrete oven floor. However, such a step may not be necessary anyway. The real problem you'll face is deterioration of the concrete from high direct heat and expansion/contraction that, combined, will lead to cracking and spalling. This is usually avoided by covering the hearth slab with firebirck which resists heat better and, since it is not a single piece, will  allow movement without cracking.

What I'd suggest as an alternative is to fabricate the slab out of concrete as is customary, but cover it with an alternative material that is cheaper than firebrick. I'm thinking granite offcuts from a countertop shop, cut to maybe 8" X 8" squares. There are a few shops around me that sell offcuts pretty cheap -- you'd just need to cut them to size. Laying them on top of your slab will protect the slab from the effects of the fire and will also make it easy to make repairs simply by popping out and replacing any damaged tile. An alternative could be to use splits -- firebrick that is about 1/2 the thickness (and about 1/2 the cost) of regular firebrick. Lay these flat like tiles over your concrete hearth instead of on their sides which is the normal practice.

Whatever route you choose, design so your total hearth masonry thickness is around 8" above an insulating layer. You may be tempted to simply start off with a concrete hearth and add some alternate surface layer later if/when problems develop. The issue you'll face there is that adding height to the hearth floor will throw your oven door opening height out of whack and make it lower than optimal, possibly affecting how the oven draws. I'd be careful about that if you're tempted to try this -- maybe place your lintel at the correct height for the oven including the added hearth layer, and lower it with something designed to be removed later if/when you add hearth material.

I've heard of using common brick for the dome -- and I'm sure it'll work -- but I doubt it'll hold up as long as a firebrick dome. So while I'll not discourage you from doing that, you should plan for maintenance sooner and more often, and design your oven so replacing dome bricks doesn't require a lot of disassembly of surrounding structure.

That said, the bottom line is that pretty much any masonry will "work", it's just a question of how long it'll work before it deteriorates so much it collapses. If you build an oven that costs you almost nothing, you've lost almost nothing if it only lasts for a couple of years -- but you've gained a couple of years of WFO. And that sounds like a winner to me!

Pittsburgh, PA

dabrownman's picture

everywhere and do not use bricks for the dome - use refractory bricks.  The heat is too much for regular bricks.    Use vermiculite (nice word for asbestos) under the bricks and over the dome fire bricks everywhere to insulate the fire bricks and keep the heat in.  You can cover the vermiculite with a premade gunite type (lightweight concrete) shell on the outside.  The gunite shell can be made by building a dome of earth on the ground and using that as a concrete form.  Leave a hole in middle at the top to put a pole in it to attach a  block and tackel to easily lift the dome in place.  The hole will allow you fille the sapce between the fire brick and the shell with vermiculite.  Paulo Soleri used the same technique to build half domes and domes for a fraction of teh price.   4" of  vermiculite should do it.  You can also Google rocket stoves on the  Internet to help design a high efficiency (well insulated) oven that will cost you an arm and a leg to fire. 

Isulation is the key to efficiency in  all things thermal .

pmccool's picture

asbestos, vermiculite itself is not asbestos.  As always, check the MSDS from the supplier.

Having seen foamglas in industrial applications, I wonder if it would be an even better choice as an insulator in WFOs.  It is available in a variety of shapes and sizes, doesn't absorb water, is easily shaped/cut, is (comparatively) lightweight, is dimensionally stable, doesn't off-gas, and stands up to temperatures that are higher than would ever be generated in a WFO.  The downsides, if any, might be cost and ability to source it.  Foamglas is much more widely used in Europe than it is here in the U.S.


breadbakin fool's picture
breadbakin fool

Foamglass is very often used as insulation in wood fired ovens, particularly underneath the hearth.  If you can get it, it makes it very easy to insulate underneath the hearth-just lay down a 4" sheet of foamglass underneath the hearth of your oven and lay down firebricks on top of that.  You could also use ceramic fiber board.  That's what I ended up using in my oven.  There is a great deal of information to be accessed about building ovens at the Yahoo brick oven group site.  Here's the link


dabrownman's picture

All vermiculite mines in the USA are contaminated with asbestos.  The one in Liddy, that used to supply 80% of the world's vermiculite, was contaminated the worst. 

Razhug's picture

Actually, I am not sure that all mines are contaminated. Taken from Wikipedia:

Asbestos contamination

Although not all vermiculite contains asbestos, some products were made with vermiculite that contained asbestos until the early 1990s. Vermiculite mines throughout the world are now regularly tested for it and are supposed to sell products that contain no asbestos. The former vermiculite mine in Libby, Montana, did have tremolite asbestos as well as winchite and richterite (both fibrous amphiboles) — in fact, it was formed underground through essentially the same geologic processes as the contaminants. A vermiculite mine in Virginia has also been found to be contaminated by asbestos.[broken citation][4]

Pure vermiculite does not contain asbestos and is non-toxic. Impure vermiculite may contain, apart from asbestos, also minor diopside or remnants of the precursor minerals biotite or phlogopite.


Also, the Vermiculite I got was from a maple syrup equipement supplier. They use it in some pumps to filter maple syrup. Additionaly, the same vermiculite is used as additives in food of organically raises cattles, so I highly doubt it contains asbestos. But as the entry in Wikipedia states, you gotta be careful about where you get it!

dabrownman's picture

All the vermiculite mines in the USA are contaminated - all of them.  We have have to import uncontaminated vermiculite from other countries since the very, very few US mines still open have to put asbestos warning lables on their packaging and it can only be used for horticulture soil enhancement or used where it is completely contained , encapsulated and cannot become fryible (microscopically airborne).

polo's picture

Vermiculite Mining and Asbestos

Vermiculite is a member of the phyllosilicate group of minerals and is found in many parts of the world. Vermiculite can contain many other minerals, including asbestos. However, not all vermiculite is contaminated with asbestos. Much vermiculite mined in the United States has been tested and does not contain asbestos fibers.

One prominent vermiculite mine in Libby, Montana was tainted with vermiculite and became one of the largest environmental disasters in the United States. The W.R. Grace vermiculite mine saw countless workers become ill and pass away, while more than a thousand residents and children in the town also were affected by the asbestos the mine released into the air.

The mine was designated as a Superfund site by the U.S. Environmental Protection Agency (EPA) in 2002. Then in 2009, EPA Administrator Lisa P. Jackson declared a public health emergency in Libby. This was the first time the EPA had declared a public health emergency, which to many public health officials, signaled the severity of the asbestos exposure in the town


Recently there have been several newspaper articles, a three-part NPR series, and a 20/20 show about asbestos contamination at the former W.R. Grace vermiculite mine located in Libby, Montana.

Most of these articles have mentioned the fact that this problem was not inherent in all vermiculite mines, but a problem of that particular Libby, Montana mine because of its unique geological formation and location. What also has been surprising and newsworthy in Libby, Montana has been the relatively recent development of health problems not just among the miners themselves, but among some of their families.

The focus of all these articles has been on the asbestos contamination at the Libby, Montana mine, not at other mines. As a result of these articles and news stories, however, many people have become confused and concerned about vermiculite in general.

Vermiculite itself does not contain asbestos; it was just the Libby, Montana mine which was contaminated because of the presence in the mountain of a secondary mineral called diopside. (To read more about diopside, go to sites like The Mineral Diopside.)

In a simplified description, in the Libby deposit there was originally biotite and diopside. With weatherization over millions of years, the biotite turned into vermiculite and the diopside turned into the asbestos.

Knowledge about the problems at Libby is not new. There have been law suits based on asbestos exposure dating back to the mid 1970's, many cases have been settled or resolved years ago, and the owner of the mine, the W.R. Grace and Company, closed the mine and most of its vermiculite processing plants thoughout North America ten years ago.

When other companies in the vermiculite industry became aware of the asbestos problems at Libby, they became very concerned and began testing for possible asbestos contamination in their operations. And today, any new source of vermiculite is tested for asbestos. These testing reports are scrutinized to make sure the vermiculite is safe, and MSDS sheets are maintained to comply with all OSHA and Community-Right-to-Know laws.



dabrownman's picture

a 3 CF bag of vermiculite at Lowes for soil attenuation last week and it contained the asbestos warning that said it should not be used unless the person has a face mask or respirator.  It is best not to take posting from asbestos and vermiculite mining .  company sites and their lobbyists.  Yes, the vermiculite is tested today, that is how we know the mines are contaminated.  The vermiculite is considered safe with proper respiratory gear but do require MSDS sheets for contamination response  and warning labels on all product sold due to asbestos contamination.  I still use it all the time though.

CountryWoodSmoke's picture

I used recycled firebricks from a night storage heater. They have been perfect, they store the heat so well and are pefrect to cook on, lots of guys use them in the UK and even New Zealand.



Thomas Parr's picture
Thomas Parr

I built my oven with old recycled "clay" bricks for the dome, and I purchased fire brick for the oven floor.  I have fired it up a number of times and it easily gets to 750 F. and holds the temperature for a long time.  At the base, under the oven floor is a layer of old beer bottles, planer chips in a slurry of  fire clay laid on 1/2" concrete board.  Above that I laid a base for the fire brick of sand and fire clay.  Refractory cement is not required between the fire brick as they appear to have a near machined edges that allow them to fit tightly side by side.  After I built the dome, I covered the bricks with heavy duty Aluminum foil, and I made a concrete mixture of sand, cement, lime' and fire clay which I troweled on, about two inches thick.  Next I made a mixture of "Perlite" (used as a soil additive in potting plants) and fire clay.  I troweled this on in two layers each about two inches thick.  I then made a plaster of cement, lime and sand and troweled it over the dome, and finally whitewashed the dome.  At the end, I made a cover over the dome.